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Trimaran Ackerman and toe out


morspeed

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Hi all,

 

I am looking for someone cleverer than me who can answer a few questions, all advice gratefully received.

 

So I am building a mini40 1.2mt model tri based loosly on a Orma 60- specifically "Banque Pop".

 

Now I am at the building of cross members stage there are a few issues I need to resolve.

 

First off, I was planning on setting ama hull at 90 degrees to waterline when main hull is just clear of water. Is this correct or to much toe out??.

 

Secondly, the issue of Ackerman I have a little more trouble getting my head around. There will be only one rudder (Main hull). I have jigged up main and ama hull so they are exactly parrallel. But it has been suggested to me that this will result in the ama bow being to far out...does this sound correct and is there some sort of formula for idiots that can work the main hull to Ama hull beam at stern and bow?. I think??, that this is all because the ama hull has a greater height from waterline compared to main hull?. At stern the ama is 7 cm higher that main hull, at lowest point of ama rocker the height difference is 2 cm...hopefully the picture will show what i mean.

 

Cheers...Gary

mini40 012.jpg

mini40 014.jpg

mini40 023.jpg

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Thankyou and thankyou SJB,

 

googled ackerman and I get bombarded by race car set up info.

 

The article you have would be gratefully received, have pm'd you.

 

Cheers..Gary

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ackermann on a multi is normally used wrt to rudder angle of attack and the argument is similar to that of motor vehicles.

 

With ama hull angle wrt the main hull - why not develop a means of altering these at the aka/ama connection. I believe they played with bow in/out on Careless Intent quite a bit but as I recall the difference between fore and aft beam connections was about 20-40 mm (rental should be able to comment) so scale that down to your model and I think you'd struggle.

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ackermann on a multi is normally used wrt to rudder angle of attack and the argument is similar to that of motor vehicles.

 

With ama hull angle wrt the main hull - why not develop a means of altering these at the aka/ama connection. I believe they played with bow in/out on Careless Intent quite a bit but as I recall the difference between fore and aft beam connections was about 20-40 mm (rental should be able to comment) so scale that down to your model and I think you'd struggle.

 

Thanks Scottie,

 

The rudder angle of attack wrt ackerman is easier for me to visualise..

 

The Ama hull adjustment is a little more difficult, but it is interesting that careless intent did not have significant difference in performance..i am not familiar with this multi, was it a tri?.

 

I do not really want adjustable beams as they will be quite small diameter and subject to a fair load so in order to make them stiff as possible they will be moulded in one piece.

 

cheers..Gary

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So far as the Toe out is concerned, if the ama bow is perpindicular to the water just as the main hull lifts out I would say that is about right.

we are going for 5 degrees of angle out on the ama.

 

the ackerman thing as far as the amas are concerned is a new one on me. I just thought you made the three centrelines parallel.

I am very interested to find out if this is not the case.

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So far as the Toe out is concerned, if the ama bow is perpindicular to the water just as the main hull lifts out I would say that is about right.

we are going for 5 degrees of angle out on the ama.

 

the ackerman thing as far as the amas are concerned is a new one on me. I just thought you made the three centrelines parallel.

I am very interested to find out if this is not the case.

 

Thankyou Mr Wolf, you confirmed my ama toe out question..i will go with this as it does seem to make the most sense.

 

As for the Ackerman question i will keep searching for an answer...really be nice to ask the designer behind G3, the answer would surely put the matter to rest.

 

Just out of interest...your tri is spectacular and i see a similar tri " Meltdown" has just been launched. Is there a set of moulds built for this design or are they all one off builds ply/foam?.

 

Whats it worth to get one of these in the water in cruising mode?.

 

Cheers...Gary

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So far as the Toe out is concerned, if the ama bow is perpindicular to the water just as the main hull lifts out I would say that is about right.

we are going for 5 degrees of angle out on the ama.

 

the ackerman thing as far as the amas are concerned is a new one on me. I just thought you made the three centrelines parallel.

I am very interested to find out if this is not the case.

 

so since your boat will be heeled over 15 degrees when the main hull lifts out (assuming min of 6 inches of chop) then your ama will be over 10 degrees away from your stated ideal of perpendicular... this makes no sense to me you just contradicted yourself?

 

Lucifer has 10 degrees, im thinking of around 7 to compromise between light airs/downwind and heavy airs/ hull flying

 

Capricorn has 5 degrees and it worked well in the light stuff and downwind but not so good in the breeze.

 

I would have a agreed with the CLs lining up, i.e no toe in at bow when looking in plan view.

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Ok, so the general consensus is all hulls parallel at CL's.

 

But here's the thing , and please feel free to correct me because you are racing these so for sure know better than me.

 

Because decks/ gunnel line are typically not a flat plane to waterline ( when hull is perpendicular to waterline)..ie... most have shear or reverse shear in the design so as soon as you tilt the ama's with toe out then the centre line of ama actually becomes an arc rather than a straight line in relation to the main hull...and so with this in mind so does the hull rocker..so once you have positioned your toe out..do you take your ama hull CL from deck bow and stern or hull bottom extremities? .

 

hope this makes sense....or not :crazy:

 

cheers..Gary

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So far as the Toe out is concerned, if the ama bow is perpindicular to the water just as the main hull lifts out I would say that is about right.

we are going for 5 degrees of angle out on the ama.

 

the ackerman thing as far as the amas are concerned is a new one on me. I just thought you made the three centrelines parallel.

I am very interested to find out if this is not the case.

 

so since your boat will be heeled over 15 degrees when the main hull lifts out (assuming min of 6 inches of chop) then your ama will be over 10 degrees away from your stated ideal of perpendicular... this makes no sense to me you just contradicted yourself?

 

Lucifer has 10 degrees, im thinking of around 7 to compromise between light airs/downwind and heavy airs/ hull flying

 

Capricorn has 5 degrees and it worked well in the light stuff and downwind but not so good in the breeze.

 

I would have a agreed with the CLs lining up, i.e no toe in at bow when looking in plan view.

 

Sam where do you get that the boat is leaning 15 degrees as the main hull lifts.

I'd guess that would be true of Lucifer and probably about that with the old amas.

 

With the new amas we are leaning way less than 15 degreees as we fly the main hull. that is the whole point of the changes !

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Ok, so the general consensus is all hulls parallel at CL's.

 

But here's the thing , and please feel free to correct me because you are racing these so for sure know better than me.

 

Because decks/ gunnel line are typically not a flat plane to waterline ( when hull is perpendicular to waterline)..ie... most have shear or reverse shear in the design so as soon as you tilt the ama's with toe out then the centre line of ama actually becomes an arc rather than a straight line in relation to the main hull...and so with this in mind so does the hull rocker..so once you have positioned your toe out..do you take your ama hull CL from deck bow and stern or hull bottom extremities? .

 

hope this makes sense....or not :crazy:

 

cheers..Gary

 

Hi Gary

 

that does make sense. But these beasties are not sailed at the same angle all the time, even though we try !

 

To your other question I willl have a mould for the amas available after construction is complete.

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tahksamycket Martin, ( i think...its been a while) very nice link.

 

Mr Wolf,

 

Perhaps the final answer I look for now is where does one determine the CL of ama to be for the purpose of measuring ama to main hull connection distances fore and afts.

 

I played around with my multi main and ama hull measurements and positioning last night...the thing is as soon as you introduce toe out to ama hull the cl of ama is no longer on the same parallel plane to main hull. The ama does not have a constant CL thru its CL cross section. (in its relationship to mainhull)

 

So for the purpose of correct distance between ama and main hull and to attach them at parallel centrelines would it be best to measure out from ama at ama waterline to mainhull center.

 

Because to take measurements from deck level stern and bow or even from lowest point of bow and futhermost aft point of hull will all give different parallel CL's

 

Getting this distance right must have a huge impact on the ability to point high.

 

Apologies for crap tech drawing terminology, naval architecture is not my strong suit.

 

Cheers...Gary

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Sam where do you get that the boat is leaning 15 degrees as the main hull lifts.

I'd guess that would be true of Lucifer and probably about that with the old amas.

 

With the new amas we are leaning way less than 15 degreees as we fly the main hull. that is the whole point of the changes !

 

I ran a couple of 3d models and found that Timberwolf used to be at 25 degrees heel when the main hull lifted, Lucifer is 20 degrees and the new timberwolf version will be 15 degrees. have you not done any calculations of dihedral angles versus righting moment versus heeling angles?

you will be suprised by the results if you do!

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The centreline doesn't move. if the amas are symmetrical then it is the line about which the other side is a mirror image.

 

Agreed, but once the ama is tilted toe out then the CL cross sections of main hull and ama are no longer on the same plane..and because the hull bottom and deck top are not symetrical to each other one must decide at what height on the side veiw of ama will you take your CL measurement to main hull?.

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Thank you Dan,

 

Eloquently put and a well described explanation of what has been troubling me.

 

Now my 1.2 mt model fits into a box rule class 1.2 x 1.2, it not a scale reproduction of a Orma 60 but is very similar in design. I am aiming for max beam and max LOA so....

 

I have spent the night fine tuning my set up and found 10 degrees of ama toe out puts the ama perpendicular to waterline when main hull is just clear of waterline.

 

So sticking with this toe out set up i then adjusted the two hulls so they were exactly parallel ,by taking main hull to ama measurements (bow and stern) from the water lines of both hulls when boat has both hulls in water. Interestingly once this was all locked in place (and this is the setup i think i will stick with) I then took measurements of bow and stern distances between ama and main hull at deck height and I discovered the CL's at decks are 1 cm closer at bow.

 

I can actually see this slightly toe in situation on the bow when looking down the length..now 1 cm is not much to be sure but if i had taken my measurements to set up ama to hull crossmember connection at deck height then the ama hull would actually end up in a 1 cm bow out situation which I am sure would effect the ability to point that little bit higher. Plus maybe when both hulls are in the water they would be essentially acting as a kinda brake.

 

Cheers...Gary

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I ran a couple of 3d models and found that Timberwolf used to be at 25 degrees heel when the main hull lifted, Lucifer is 20 degrees and the new timberwolf version will be 15 degrees. have you not done any calculations of dihedral angles versus righting moment versus heeling angles?

you will be suprised by the results if you do!

 

No sam. the angle when we fly the main hull is actually just slightly less than 5 degrees

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Awesome excellent way of working it out. 1cm in 120 sounds pretty significant, it would be 50mm on a 6m tri, which is pretty big, considering the lengths we go to to get parallel hulls on similar sized cats.

 

I guess it follows on from your experiment, that if you are going to have canted amas - where then is the best place to put your boards and rudders. Ama or Aka, or all 3 and raise and lower depending on tack and heel angle!

 

A smarter person could have probably worked all this out with 3d modelling...I have to work with old fashioned drawing tools.

 

Boards and rudder positions etc...are not such a concern for me as these models use one centre board and one rudder all in main hull, and the mechanics of raising / lowering foils on a model adds to much weight to be an option (presently) ...however ...there is a real issue (I think ) with main hull and therefore rudder and centreboard, and how these foils interface with a ama hull that is not parallel to them when flying a hull.

 

I think my set up will now work ok, but i am sure there is room for improvement.

 

Cheers...Gary

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